Cyclic AMP plays an important role in the regulation of hormonal actions and of cell growth. Cyclic nucleotide phosphodiesterase (PDE), which catalyzes the hydrolysis of cyclic AMP, exists in several molecular forms in many tissues. A dependent form of PDE requires a Ca ions binding protein to reach maximal activity through the formation of Ca ions -activator-enzyme complex. By the selective inhibition or activation of the PDE isozymes, it may be possible to alter the course of diseases characterized by an abnormal metabolism of cyclic nucleotides. BIBLIOGRAPHIC REFERENCES: Liu, Y.P., and Speer, R.J.: The carboxyl- and amino-terminal amino acids and other properties of L-asparaginase from Escherichia Coli. J. Form. Med. Assoc. 75: 168, 1976. Liu, Y.P., and Cheung, W.Y.: Cyclic 3', 5'-nucleotide phosphodiesterase. Ca ions confers more helical conformation to the protein activator. J. Biol. Chem. 251: 4193, 1976.